1. Field of the Invention
The present invention relates to devices for reading a bar code, and in particular to a bar code reading camera which is remotely disposed with respect to the bar code to be read and which operates so as to substantially continuously scan a region through which a plurality of bar codes to be read are moved.
2. Description of the Prior Art
Bar code readers having a sufficient depth of field to read bar codes at a distance are known in the art and employ a laser tube as a light source and are commonly referred to as laser scanners. Laser scanners generally operate by directing a laser beam at a rotating drum or multi-faced mirror or hologram to scan the beam across a bar code. The reflected scattered light is incident on a photocell which generates a corresponding voltage so that a voltage pattern with respect to the distance along the length of the bar code is developed. Because the rotational speed of the drum or mirrors or hologram is known, the reflectivity-length relationship can be converted to a voltage-time relationship, which is used to decode the information contained in the bar code. Because of the capability of reading bar codes at a distance, laser scanners are frequently employed in industrial applications to read bar codes on a large number of items moving past the reader, such as on a conveyor.
Such laser scanners have several disadvantages. The rotating drum or mirrors and the drive means therefor involve moving parts subject to wear and requiring servicing. The laser tube used in such scanners has a limited life and the devices embodying such tubes are complex and expensive. Very accurate means are required to correlate the rotation of the drum or mirror with the decoding circuitry in order to precisely identify the reflection pattern with the distance along the length of the bar code, making laser scanners sensitive to vibration. Finally because laser scanners necessarily employ highly concentrated coherent radiation, which is directed across a relatively open area, such devices pose a safety hazard to workers in the area.
Portable bar code readers which employ a light source other than a laser are also known. Such conventional portable bar code readers, however, do not have a depth of field sufficient to scan a bar code which is disposed some distance from the optical scanning means. Such conventional devices generally utilize a wand or other hand held instrument which is manually moved across the bar code in contact therewith or in close proximity thereto, the reflected light being incident on a photodiode array which is electronically scanned. Such conventional devices also generally employ an illumination means in the form of a flash or strobed light source so that the bar code is only momentarily illuminated. Such portable units, in addition to a compact size, sacrifice processing speed and depth of field in order to minimize the number and size of the components comprising the device.
Because of the lack of suitable depth of field and the low power, flashed illumination associated with such conventional bar code readers, those devices are not suitable for industrial use in, for example, inventory control, which requires substantially continuous scanning of a large number of successive bar codes. In such applications, it is not practical to manually pass the illumination source over each individual bar code, as is required by portable reader models. Moreover, such portable models cannot be easily adapted to existing assembly line installations wherein the frequency with which the different bar codes are to be read is not known in advance and therefore cannot be precisely coordinated with the flash necessary in such conventional readers to illuminate the bar code.